Where plates collide

Where plates collide (Denali, Aconcagua, Kanchenjunga), impressive mountains form. In collision, one plate usually slides beneath the other, plunging-in the so-called subduction zone-as much as four hundred miles. The material carried down there tends to melt, and to rise as magma, reaching the surface in volcanic form, as in the Cascade Range, the Andes, the Aleutians, and Japan. Yellowstone, with all its magmatic products and bubbling sulphurs, attracts special attention in the light of this story, because Yellowstone is eight hundred miles from the nearest plate boundary. When the theory of plate tectonics developed, it asked as well as answered questions-and not a few of the questions were inconvenient to the theory. Many had to do with volcanism. For example, why was the island of Hawaii pouring out lava in the dead center of the Pacific Plate? Similarly, if volcanoes were the products of subduction zones, where was the nearest subduction zone to the Tibesti Mountains of Saharan Chad? The Tibesti massif-a couple of thousand kilometres from the leading edge of the African Plate-consists of shield volcanoes like Mauna Kea and Mauna Loa. Where was the closest subduction zone to the chain of peaks that culminates in Mt. Cameroon, a stratovolcano fifteen hundred miles from the nearest plate boundary of any kind? Moreover, some of the fine old conundrums of geology-problems that antedated the plate-tectonics revolution-remained standing in zakelijke energie vergelijken its aftermath. What could explain the Canadian Shield? The South American Shield? The South African Shield? How could so much Precambrian rock lie close to sea level and not have been buried in a thousand million years? What, in recent time, had lifted the platform of the Rockies, causing their exhumation? Why were Love and I, there on the platform, not at sea level? What had lifted the Colorado Plateau, subjecting it to incision by canyon-cutting rivers? What explained flood basalts? Plate tectonics seemed to have no relevance to them. With plate the01y, you would think you could predict the sedimentary history of continents, but you couldn’t. Why were continental basins-the Michigan Basin, the Illinois Basin, the Williston Basin-several kilometres deep? If you expect a shieldlike situation as the ultimate scene, what could zakelijke energie explain these anomalous deep basins? Oil people wanted to know most of all. They asked plate theorists, “What does plate tectonics tell us about these basins?” The answer was “Nothing.”